Project Summary The long range goal of our research program is to quantify intercellular coupling in acute and chronic heart dis- ease, which will lead to dramatically improved descriptions of arrhythmia substrates. Maintenance of coupling strength between cardiac myocytes is essential for normal electrical activity. That strength depends primarily on intracellular gap junction coupling and on interstitial coupling in the fibrillar collagen network. Each flow path is well recognized and there is considerable evidence to suggest microimpedance increases promote arrhyth- mias. Until recently, no procedure or standard instrument for measuring intracellular and interstitial impedances has been available, so information on their magnitudes is limited. The major near-term goal of this proposal is to apply our newly developed theoretical and experimental approach to measuring intracellular and intersti- tial microimpedances. That approach involves use of a precisely designed but very small set of electrodes for multisite stimulation and mathematical evaluation of the underlying tissue impedances from the set of recorded voltages. It has the potential to become a straightforward component of cardiac electrophysiologic study because no intracellular access is required to obtain the microimpedances. This RO1 scale Bioengineering Research Grant, which is responsive to PA-07-279, builds on expertise developed in our R21 scale Exploratory Bioengi- neering Research Grant. Studies will focus on rabbit left ventricular epicardium, providing quantitative details for this specific preparation. This revised application has the following aims: (1) to transition our microimpedance measurement approach from its developmental framework for application to heart preparations; (2) to measure spatial variabilities in directional interstitial microimpedances under normal conditions and during interstitial com- partment size adjustments; (3) to measure spatial variabilities in directional intracellular microimpedances under normal conditions and during interventions targeting gap junctions; and (4) to quantify microimpedance changes during development of ischemia-like conditions and subsequent reperfusion. The project is significant because the electrical properties of intracellular and interstitial pathways for cardiac excitation as they exist in vivo will be carefully measured on the size scale where propagation failure contributes to arrhythmia initiation and mainte- nance. Completion of the project's near-term objectives will have a longer-term impact because it will allow us to focus on adapting our approach for use with animal models of chronic heart disease, preparations that include interfaces between different tissue types and simultaneous stimulation and recording from multiple microelectrical mechanical systems arrays to obtain regional microimpedance measurements. Thesaurus Terms: electrical impedance, electrical measurement, heart electrical activity, interstitial, intracellular, method development, cardiac myocyte, electrical conductance, membrane model, myocardium, electrode.